Hindawi Publishing Corporation BioMed Research International Volume 2015, Article ID 754934, 9 pages http://dx.doi.org/10.1155/2015/754934

Review Article : Photoautotrophic Microbial Factories for the Sustainable Synthesis of Industrial Products

Nyok-Sean Lau,1 Minami Matsui,2 and Amirul Al-Ashraf Abdullah1,3

1 Centre for Chemical Biology, Universiti Sains Malaysia, 11900 Bayan Lepas, Penang, Malaysia 2Synthetic Genomics Research Team, RIKEN Centre for Sustainable Resource Science, Biomass Engineering Research Division, Yokohama, Kanagawa 230-0045, Japan 3School of Biological Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia

Correspondence should be addressed to Amirul Al-Ashraf Abdullah; [email protected]

Received 3 April 2015; Accepted 16 June 2015

Academic Editor: Zongbao K. Zhao

Copyright © 2015 Nyok-Sean Lau et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Cyanobacteria are widely distributed Gram-negative with a long evolutionary history and the only prokaryotes that perform plant-like oxygenic photosynthesis. Cyanobacteria possess several advantages as hosts for biotechnological applications, including simple growth requirements, ease of genetic manipulation, and attractive platforms for carbon neutral production process. The use of photosynthetic cyanobacteria to directly convert carbon dioxide to biofuels is an emerging area of interest. Equippedwiththeabilitytodegradeenvironmentalpollutantsandremoveheavymetals,cyanobacteriaarepromisingtoolsfor bioremediation and wastewater treatment. Cyanobacteria are characterized by the ability to produce a spectrum of bioactive compounds with antibacterial, antifungal, antiviral, and antialgal properties that are of pharmaceutical and agricultural significance. Several strains of cyanobacteria are also sources of high-value chemicals, for example, pigments, vitamins, and enzymes. Recent advances in biotechnological approaches have facilitated researches directed towards maximizing the production of desired products in cyanobacteria and realizing the potential of these bacteria for various industrial applications. In this review, the potential of cyanobacteria as sources of energy, bioactive compounds, high-value chemicals, and tools for aquatic bioremediation and recent progress in engineering cyanobacteria for these bioindustrial applications are discussed.

1. Introduction of oxygenic photosynthesis but some cyanobacterial species can switch to sulfide-dependent anoxygenic photosynthesis Cyanobacteria, also referred to as blue-green algae, are the [6]. In dark or under anoxic conditions, cyanobacteria can oldest photosynthetic organisms on earth that originated perform fermentations for energy generation [7]. Some fila- approximately 2.6–3.5 billion years ago [1]. Indeed, the origin mentous cyanobacteria have evolved specialized cells known of photosynthetic organelle in eukaryotes is thought to have as heterocysts to carry out nitrogen fixation [8]. possibly arisen by the process of endosymbiosis between a At present, many bioindustrial processes rely on the phagotrophic host and a cyanobacterium [2]. Cyanobacteria fermentations of heterotrophic bacteria to produce various are morphologically diverse and exist in different forms fine chemicals such as vitamins, enzymes, and amino acids. including unicellular, filamentous, planktonic or benthic, and Nevertheless, the economic viability of these production colonial (coccoid) ones [3, 4]. They are by far the most schemes is limited by the cost of carbon substrates used in the widespread occurring photosynthetic organisms. They can fermentation processes. Cyanobacteria, endowed with pho- thrive in a wide range of ecological habitats, ranging from tosynthesis system to fix carbon dioxide into reduced form, marine, freshwater, to terrestrial environments. Cyanobacte- are ideal biosynthetic machinery for sustainable production ria are also well known for their ability to perform different of various chemicals and biofuels. Unlike heterotrophic modes of metabolism and the capacity to switch rapidly bacteria, cyanobacteria require only sunlight, carbon dioxide, from one mode to another [5]. All cyanobacteria are capable water, and minimal nutrients for growth, eliminating the cost 2 BioMed Research International of carbon sources and complex growth media. Sunlight is the up to 550 mg/L ethanol was achieved in the engineered most readily available and inexpensive resource on earth and Synechocystis sp. [32]. Further engineering of Synechocystis the use of cyanobacteria for the production of fine chemicals sp. by overexpressing endogenous alcohol dehydrogenase and biofuels from solar energy offers a greener path for the and disrupting polyhydroxyalkanoate biosynthetic pathway synthesis process. Equipped with superior photosynthesis increased ethanol production up to 5500 mg/L [12](Table 1). capabilities, cyanobacteria have higher photosynthesis and Isobutanol and 1-butanol are considered as better substitutes biomass production rates compared to plants and can convert for gasoline compared to ethanol as they have greater energy up to 3–9% of the solar energy into biomass compared to density, being less corrosive and less volatile [33]. Using a ≤0.25–3% achieved by crops, for example, corn, sugar cane similar genetic modification approach, direct photosynthetic [9]. They also require less land area for cultivation than production of isobutanol in cyanobacteria is also feasible. The terrestrialplant,reducingthecompetitionforarableland introduction of an artificial isobutanol biosynthesis pathway with crops intended for human consumption. Cyanobacteria into Synechococcus elongatus PCC 7942 had resulted in the utilize carbon dioxide, a type of greenhouse gases, during production of isobutyraldehyde and isobutanol up to 1100 and photosynthesis and help to achieve a carbon neutral pro- 450 mg/L, respectively [13]. duction process. Being prokaryotes, cyanobacteria possess More than 14 genera of cyanobacteria are known to relatively simple genetic background that eases manipulation produce hydrogen under various culture conditions and they [10]. In addition, the residual cyanobacteria biomasses that include Anabaena, Aphanocapsa, Calothrix, Microcystis, Nos- are left over after high-value products extraction can be used toc,andOscillatoria [15, 34–36]. In heterocystous or filamen- as animal feed or converted into organic fertilizer. tous cyanobacteria, hydrogen gas is produced as a byproduct Considering the aforementioned inherent merits of of nitrogen fixation under nitrogen limiting growth con- cyanobacteria, they are one of the attractive candidates for ditions. In cyanobacteria, the production of hydrogen gas use in diverse biotechnological application. With the recent is also facilitated by the reversible activity of hydrogenases advances in genetic and metabolic engineering technologies enzymes. Two distinct types of hydrogenases are present in and the availability of more than 300 cyanobacterial genome different cyanobacterial species: uptake hydrogenases that sequences, there is significant progress in research directed oxidize oxygen; bidirectional or reversible hydrogenases that towards realizing the full potential of these photosynthetic can both take up or produce hydrogen [37]. The efficiency bacteria. Cyanobacteria have gained considerable attention in of hydrogen production in some cyanobacteria is limited recent years for their possible use in agriculture, nutraceu- by the extreme oxygen sensitivity of hydrogenases and the ticals, effluent treatment, and the production of biofuels, tendency for [NiFe] hydrogenases to thermodynamically various secondary metabolites including vitamins, toxins, favor hydrogen uptake [38]. Increased production of hydro- and enzymes. In this paper, the recent progress in developing gen is achievable by blocking pathways that compete for cyanobacteria for various potential applications in biotech- reductant consumption with hydrogenases. The disruption nology is discussed. of ldhA gene that is responsible for NADH consumption in lactate production in Synechococcus sp. PCC 7002 had + 2. Potential Applications of Cyanobacteria as resulted in a significant increase in the NADH/NAD ratio Energy Sources and a concomitant fivefold increase in hydrogen produc- tion by the native bidirectional [NiFe] hydrogenase [39]. Anticipation of depletion of fossil fuel resources and global In another example, enhanced hydrogen production in S. warming have spurred vigorous research initiatives aimed elongatus PCC 7942 was obtained through heterologous at developing carbon neutral alternatives to supplement or expression of exogenous [FeFe] hydrogenases (HydA) from replace fossil fuels. Several alternatives to current fossil fuels Clostridium acetobutylicum [16]. The expression of [FeFe] have been proposed and they include ethanol, 1-butanol, hydrogenases that thermodynamically favor hydrogen pro- isobutanol, hydrogen gas, and alkanes. ductionrelativeto[NiFe]hydrogenasecanmodulateredox The production of ethanol via biological route has flux in the heterologous host, resulting in higher hydrogen received widespread attention in recent years. Traditionally production. Metabolic engineering of cyanobacteria by redi- a two-step route to first collect plant-derived biomass and recting glycogen catabolism through the oxidative pentose subsequent conversion of the biomass to fuels by microbial pathway was found to enhance intracellular NADPH con- fermentation is employed [30]. This indirect production centrations and consequently improve the hydrogen yield. scheme is inefficient in the conversion of biomass to fuels In the glyceraldehyde-3-phosphate (gap1) gene deletion and + [31] and thus there are increasing interests in the use of NAD -glyceraldehyde-3-phosphate (GAPDH-1) overexpres- photosynthetic microbes to directly convert carbon dioxide sion strains of Synechococcus sp. PCC 7002, 2.3-fold and to fuels. Although some cyanobacterial strains naturally 3-fold increase in hydrogen production, respectively, were produce low level of ethanol as a byproduct of natural obtained compared to the wild type [40]. fermentation, it is necessary to enhance the production effi- Some cyanobacteria are known to synthesize alkanes ciency of cyanobacteria to reach an economically viable level. or alkenes that have desirable properties for combustion. An attempt to introduce the pyruvate decarboxylase (pdc) Studies on the alkane biosynthetic pathways in cyanobacteria and alcohol dehydrogenase II (adh) genes from Zymomonas revealed that the two important enzymes acyl-acyl carrier mobilis into the chromosome of Synechocystis sp. PCC 6803 protein reductase and aldehyde decarbonylase are responsi- was reported [11](Figure 1). Photosynthetic production of ble for the conversion of fatty acid metabolism intermediates BioMed Research International 3

aar adc Fatty acids Fatty aldehyde Alkanes

Acetyl-CoA PHB CO2 3-PGA PEP

alsS ilvC ilvD kivd RuBP Calvin G3P Pyruvate 2-Ketoisovalerate Isobutyraldehyde cycle pdc adh DXP F6P Acetaldehyde Isobutanol adh NADPH

FNR Ethanol hydA ispS Hydrogen gas Fd HMBPP DMAPP Isoprene Light Light

Ctb6f PSII − e PSI

+ + H2O 2 +1/2 H H O2

Figure 1: A schematic representation of biochemical pathways for various industrial products synthesis in cyanobacteria. 3-PGA, 3- phosphoglycerate; aar,aldehydedecarbonylase;adc, alcohol dehydrogenase; alsS, acetolactate synthase; F6P, fructose-6-phosphate; FNR, + ferredoxin NADP reductase; G6P, glucose-6-phosphate; HydA, [FeFe] hydrogenase; ilvD, dihydroxy-acid dehydratase; ilvC, acetohydroxy acid isomeroreductase; pdc, pyruvate decarboxylase; PEP, phosphoenolpyruvate; PHB, polyhydroxybutyrate.

Table 1: Biofuels production in cyanobacteria.

Cyanobacteria Compound Production References Synechococcus elongatus PCC 7942 Ethanol 230 mg/L [11] Synechocystis sp. PCC 6803 Ethanol 5500 mg/L [12] Synechococcus elongatus PCC 7942 Isobutanol 450 mg/L [13] Synechococcus elongatus PCC 7942 Isobutyraldehyde 1100 mg/L [13] Synechococcus elongatus PCC 7942 1-Butanol 29.9 mg/L [14] − − Anabaena sp. PCC 7120 Hydrogen 2.6 𝜇mol mg 1 chl a h 1 [15] − − Anabaena cylindrica IAM M-1 Hydrogen 2.1 𝜇mol mg 1 chl a h 1 [15] − − commune IAM M-13 Hydrogen 0.25 𝜇mol mg 1 chl a h 1 [15] − − Synechococcus elongatus PCC 7942 Hydrogen 2.8 𝜇mol mg 1 chl a h 1 [16] Synechococcus sp. PCC 7002 n-Alkanes 5% dry cell weight [17] Synechocystis sp. PCC 6903 Fatty acids 197 mg/L [18]

to alkanes or alkenes [41, 42]. Heterologous expression of biofuel precursors including alkanes, fatty acids, and wax genes encoding these two enzymes had conferred the ability esters from fatty aldehydes was achievable in Synechocystis toproduceandsecretealkaneinEscherichia coli [43]. In sp. PCC 6803 through pathway engineering. Overexpression addition, the expression of acyl-acyl carrier protein reductase ofacyl-ACPreductase,theenzymethatconvertstheend and aldehyde decarboxylase genes from S. elongatus PCC product of fatty acid biosynthesis into acyl aldehyde, resulted 7942 in Synechococcus sp. PCC 7002 was found to increase in a significant improvement in fatty acids production in alkane production in the heterologous host [17]. In another Synechocystis sp., in addition to alkane production [45]. In example, the overexpression of both acyl-acyl carrier pro- another study, the coproduction of alkanes and 𝛼-olefins was tein reductase and aldehyde-deformylating oxygenase from observed in Synechococcus sp. NKBG15041c by expressing the several cyanobacterial strains was shown to increase alka- acyl-acyl carrier protein reductase/aldehyde-deformylating nes/alkenes yield by twofold [44]. The production of various oxygenase pathway genes from S. elongatus PCC 7942 [46]. 4 BioMed Research International

Table 2: Potential uses of cyanobacteria in bioremediation of wastewater.

Cyanobacteria Types of wastewater Compounds removed References − 3− Phormidium bohneri Industrial effluent (cheese factory) NO3 and PO4 [19] + 3− Phormidium bohneri Swine manure effluent NH4 and PO4 [20] − 3− Oscillatoria sp. Activated sludge effluent NO3 and PO4 [21] Schizothrix calcicola, Phormidium subfuscum, − 3− Synthetic wastewater NO3 and PO4 Phormidium tenue,andOscillatoria sp. Spirulina indica, Spirulina maxima,andSpirulina Synthetic heavy metal solution Nickel and zinc [22] platensis + Copper, iron, NH4 ,and Anabaena doliolum, Chlorella vulgaris Synthetic wastewater − [23] NO3 Industrial wastewater [chromium Nostoc sp. PCC 7936 Chromium (VI) [24] (VI) plating industry] Anabaena oryzae, Anabaena variabilis,and Mixed domestic-industrial Organic matter, copper, and [25] Tolypothrix ceytonica wastewater zinc Synechocystis sp. PUPCCC 64 Synthetic insecticide solution Chlorpyrifos [26] Westiellopsis prolifica, Nostoc hatei,and Carbofuran, chlorpyrifos, and Synthetic insecticide solution [27] Anabaena sphaerica endosulfan Synechocystis sp. PUPCCC 64 Synthetic herbicide solution Anilofos [28] Anabaena oryzae, Nostoc muscorum,and Synthetic pesticide solution Malathion [29] Spirulina platensis

3. Role of Cyanobacteria in The use of immobilization to entrap cyanobacteria in matri- Aquatic Bioremediation ces (agarose, carrageenan, chitson, alginate, and polyurethane foam) can help to solve the harvesting problem. In addition, In recent years, biological waste treatment systems, in par- immobilized cyanobacteria show higher efficiency in nutrient ticular the use of cyanobacteria in wastewater treatment, or metal removal compared to their free-living counterpart. have attracted considerable scientific and technical interest. Immobilization of Anabaena doliolum was shown to increase Cyanobacteria are characterized by the ability to oxidize oil its copper and iron ions removal capacity in the order of components, complex organic compounds, and accumulate 45 and 23% higher than its free-living counterpart [23]. metalions,forexample,Zn,Co,andCu[47]. Thus, cyanobac- Similarly,theefficiencytotakeupnitrogenousandphospho- teria are promising tool for the secondary treatment of urban, ruscompoundswasenhancedinimmobilizedChlorella and agricultural, or industrial effluents (Table 2). Anabaena [23]. The potentials of cyanobacteria in the removal of nutri- In natural environment, many cyanobacteria form sym- ents from wastewater rich in nitrogenous and phospho- biotic associations with other aerobic or anaerobic microor- rus compounds have been demonstrated. The ability to ganisms. It is interesting to note that cyanobacterial mats remove nitrogenous and phosphate ions from wastewater was including Oscillatoria, Synechocystis,andPleurocapsa have observed in cyanobacteria such as Oscillatoria, Phormidium, been shown to aid in the degradation of hydrocarbons present Aphanocapsa,andWestiellopsis [19–21, 48]. The biomass in oil. Although cyanobacteria are not directly responsible of Spirulina strains contains different functional groups, for the degradation of hydrocarbons, they facilitated the for example, carboxyl, hydroxyl, sulfate, and other charged degradation process by providing oxygen and nutrients to the groups that are important for metal binding. They have great associated oil-degrading bacteria [50]. A consortium com- potentials in metal pollution control and the biosorption prising Phormidium, Oscillatoria, Chroococcus,andtheoil- of zinc and nickel by several strains of Spirulina,namely, degrading bacterium, Burkholderia cepacia, was successfully Spirulina indica, Spirulina maxima,andSpirulina platensis, developed and employed on a rotating biological contactor to was investigated recently [22]. Pesticide application is known efficiently degrade petroleum compounds [51]. to have negative impacts on soil ecology and cyanobacte- riahavebeenreportedtoaccumulateanddetoxifythese 4. Cyanobacteria as Potential Bioactive pesticides. Previous studies demonstrated that Synechocystis Compounds Sources sp. PUPCCC 64, Westiellopsis prolifica, Nostoc hatei,and Anabaena sphaerica are able to degrade organophosphorus Cyanobacteria have been identified as sources of bioac- or organochlorine insecticides in the aquatic environment tive compounds with interesting biological activities, for [26, 27]. example, antibacterial, antifungal, antiviral, antialgal, anti- One of the drawbacks that limit the practical applications cancer, anti-inflammatory, and so forth. These bioactive of cyanobacteria in wastewater treatment is the difficulty in compounds include lipopeptides (40%), amino acids (5.6%), the separation of biomass from the effluent before discharge [49]. fatty acids (4.2%), macrolides (4.2%), and amides (9%) [4]. BioMed Research International 5

The excretion of bioactive compounds by cyanobacteria into the major carotenoids accumulated by cyanobacteria are the aquatic environments is possible allelopathy strategy beta-carotene, zeaxanthin, nostoxanthin, echinenone, and used by cyanobacteria to outcompete other microorgan- canthaxanthin. These pigments are commonly used as food isms within the same ecosystem [52]. These allelopathic colorants, food additives, and supplements for human and compounds include alkaloids, cyclic peptides, terpenes, and animal feeds. Carotenoids are well known for their antioxi- volatile organic compounds. Cyanobacterial allelochemicals dant properties and their possible role in the prevention and are found to exhibit inhibitory effects toward the growth, control of human health and disease conditions, for example, photosynthesis, respiration, carbon uptake, and enzymatic cancer, cardiovascular problems, cataracts, and muscular activity of algae as well as induce oxidative stress in algae dystrophy, has been reported [62]. [53]. Todate, several allelochemicals from cyanobacteria have Some marine cyanobacteria are valuable sources of vita- been identified and they include cyanobacterin produced by minsandtheyarebeingusedforthelarge-scaleproductionof hofmanni, enediyne-containing photosystem II vitamins of commercial interest such as vitamins B and E. For inhibitor synthesized by Fischerella muscicola, hapalindoles example, Spirulina (Arthrospira)isknowntobearichsource thatareisolatedfromHapalosiphon and Fischerella spp., and of vitamin B12, beta-carotene, thiamine, and riboflavin. It nostocyclamides from Nostoc sp. These antialgal compounds is marketed in the form of powder, granules, tablets, or synthesized by cyanobacteria can be developed as alternative capsules and commercially available Spirulina tablets contain tosyntheticalgicidesthatareusedtocontrolharmfulalgal up to 244 𝜇g of vitamin B12 per dry weight [63]. Cyanobac- blooms. teria are found to secrete a broad spectrum of enzymes Extensive screenings are presently undergoing to search that can be exploited for commercial applications. These for new antibacterial compounds from cyanobacterial industrially important enzymes include protease, amylase, extractsthatcanbepotentiallydevelopedasnewdrugsor and phosphatases. Proteases are predominantly used for food antibiotics. It has been reported that cyanobacterial extracts processing industries; alpha-amylases are extensively used in of Westiellopsis prolifica ARM 365, Hapalosiphon hibernicus starch industries and phosphatases and acid phosphatases are ARM 178, Nostoc muscorum ARM 221, Fischerella sp. ARM widely used as diagnostic markers [64]. 354, and Scytonema sp. showed antibacterial activity against Pseudomonasstriata,Bacillussubtilis,E. coli,andBradyrhizo- 5.2. Isoprene. Isoprene is an energy rich hydrocarbon that bium sp. [54]. In another study, Anabaena sp. was shown to ispotentiallyabiofuelandanimportantfeedstockinthe exhibit antibacterial activity against Staphylococcus aureus, synthetic chemistry industry. It is used industrially as the E. coli, Pseudomonas aeruginosa, Salmonella typhi,andKleb- starting material to make synthetic rubber. Although iso- siella pneumonia [55]. Noscomin, a diterpenoid compound prene is naturally synthesized and released by many herba- isolated from Nostoc commune, showed antibacterial activity ceous, deciduous, and conifer plants into the surrounding against Bacillus cereus, Staphylococcus epidermidis,andE. coli environments, it is impractical to harvest isoprene from plant at MIC values comparable with those obtained for standard in large-scale. The production of isoprene by photosynthetic drugs [56]. Additionally, cyanobacteria produce a broad cyanobacteria through heterologous expression of isoprene spectrum of compounds with antifungal activity. For exam- synthase (IspS)fromPueraria montana was demonstrated ple, nostofungicide isolated from N. commune is effective [65]. The accumulation of ∼50 𝜇g isoprene per g dry cell against Aspergillus candidus;norharmanefromNostoc insu- weight per day was achievable in recombinant Synechocys- 󸀠 lare and 4,4 -dihydroxybiphenyl from Nodularia harveyana tis sp. PCC 6803 transformed with codon optimized IspS showed potent antifungal activity against Candida albicans gene. In recent work, the introduction of isoprene synthase [57, 58]. Antiviral activity of extracts or compounds isolated together with the mevalonic acid pathway genes was found to from cyanobacteria has also been reported. Cyanovirin-N, increase photosynthetic isoprene production yield to approx- N, and sulfoglycolipid isolated from Nostoc imately 2.5-fold compared with cyanobacteria transformed ellipsosporum, Scytonema varium,andScytonema sp., with IspS gene only [66]. respectively, are shown to exhibit potent antiviral activity against human immunodeficiency virus (HIV) [59–61]. 5.3. Biopolymers. Polyhydroxyalkanoate (PHA) is a type of biodegradable polymer that can serve as substitute for 5. Cyanobacteria as Sources of petroleum-based plastics and a biocompatible material that Value-Added Products has promising applications in biomedical or pharmaceutical field. Several cyanobacteria including Aphanothece sp. [67], 5.1. Pigments, Vitamins, and Enzymes. Cyanobacteria are Oscillatoria limosa [68], some species of the genus Spirulina known to produce various fine chemicals and there are [69, 70], and the thermophilic strain Synechococcus sp. considerable interests in the production of these chemicals MA19 [71] are natural producers of PHA. However, the from cyanobacteria on a commercially viable scale. Two PHA yield obtained from direct photosynthetic production important cyanobacterial pigments, phycobiliproteins and in cyanobacteria is low (<10%). Nutrient-limiting culture carotenoids, are extensively used in bioindustry and have conditions and carbon feedstocks addition were found to high commercial value. Phycobilisomes are the major light- enhancePHAaccumulation.Theapplicationsofphosphorus- harvesting complexes present in cyanobacteria and they deficiency, gas-exchange limitation culture conditions as well consist mainly of phycobiliproteins such as phycocyanin, as fructose and acetate addition had resulted in remarkable allophycocyanin, and phycoerythrin. On the other hand, increases in PHA accumulation up to 38% of the dry cell 6 BioMed Research International weight in Synechocystis sp. PCC 6803 [72]. In another attempt by approximately twofold in the genetically engineered strain to enhance photosynthetic PHA production in Synechocystis [77]. sp. PCC 6803, the introduction of an acetoacetyl-CoA syn- Light availability plays pivotal role in determining the thase that catalyzes the irreversible condensation of acetyl- growth of photoautotrophs; thus, selective pressure has CoA and malonyl-CoA to acetoacetyl-CoA was found to favored adaptations that maximize light capture in photosyn- have a positive impact on PHA production [73]. The highest thetic biological systems to compete for sunlight. However, PHA production obtained from photosynthetic cultures of under high light conditions, the excessive capture of photons genetically Synechocystis sp. was 14 wt%. High PHA accu- bycellsinthesurfacelayerofcyanobacterialculturesis mulation of up to 52% dry cell weight was demonstrated lost through thermal dissipation, limiting the availability in marine cyanobacterium Synechococcus sp. PCC 7002. The of light to cells underneath. To address this issue, attempt Synechococcus sp. was transformed with plasmid carrying to artificially reduce the light-harvesting antenna size was Cupriavidus necator PHA biosynthetic genes using recA com- carried out. This strategy is designed to enable cells at the plementation as selection pressure for plasmid stability [74]. culture surface to capture only the amount of light that Interestingly, the ability of (S)- and (R)-3-hydroxybutyrate they require and allow greater penetration of excess light molecules synthesis and secretion from genetically engi- into the culture. In a phycocyanin-deficient Synechocystis neered Synechocystis sp. PCC 6803 cells was also reported strain, a substantial improvement in photosynthetic activ- [75]. ity and biomass production was obtained relative to their wild-type counterparts [78]. As oxygenic photosynthesis in 6. Challenges and Outlook cyanobacteria effectively utilizes photons in the waveband of 400–700 nm, the remainder of the energy is lost as heat. In Considering the great potentials of cyanobacteria in diverse addition, the photosystems in most photosynthetic organ- biotechnological applications, it would be of scientific, tech- isms compete for solar radiation with the same wavelengths, nological, and industrial interests to translate theses photo- reducing the overall energy conversion efficiency. One of the synthetic biological systems’ potential into realizable large- interesting strategies to increase photosynthetic efficiencies scale production. Although a broad spectrum of natural suggests the engineering of one of the two photosystems to products synthesized by cyanobacteria has potential bioin- extend the absorption maxima to ∼1100 nm [79]. dustrial applications, the underlying challenge in commer- While biotechnological production potentials of cialization is to achieve production yields that meet realistic cyanobacteria have attracted considerable interest, high scalable production. yield synthesis of industrial products from cyanobacteria The potential uses of cyanobacteria as hosts for the pro- remains challenging. In biofuels synthesis, the production duction of various carbon-containing industrial products, for ability of cyanobacteria is likely limited by the low level of example, ethanol, isobutanol, hydrogen gas, and alkanes, have cells tolerance to biofuel. RNA-sequencing was carried out been extensively explored for the past ten years [13, 16, 17, 32, to determine the transcriptome profile of Synechocystis sp. 33, 39]. In this production model, the cyanobacterial cells can PCC6803 cultivated under different concentrations of exoge- be seen as photosynthetic microbial cell factories that harvest nous n-butanol. The study revealed that the overexpression solar energy to fix carbon dioxide into products of interest. As of several candidate proteins, particularly the small heat the carbon content of the synthesized compound originates shock protein, HspA, improved the tolerance of Synechocystis from the carbon dioxide fixed during photosynthesis, the sp. to butanol [80]. To understand the metabolomics changes productivity of the system depends on the efficiency of related to butanol stress, metabolomics profile of Synechocys- carbon dioxide fixation. On the other hand, photosynthetic tis sp. PCC 6803 cultivated under gradual increase of butanol efficiencies are affected by the efficiency of solar photon concentration was determined. The identification of metabo- captureandtheconversionofcapturedphotontochemically lites that were differentially regulated during the evolution process provides metabolomics basis for further engineering stored energy [33]. Although cyanobacteria are equipped of cyanobacteria to improve their product tolerance [81]. with photosynthetic machineries that are two- to threefold Although most strategies implemented to engineer more efficient in solar energy conversion than that of crop cyanobacteria as microbial cell factories for various high- plants, the efficiencies are still low with yield less than 10% value products synthesis focus on local pathway optimization, [9]. Genetic engineering approaches aiming at improving system biology approaches (e.g., transcriptomes, proteomics, the photosynthetic efficiencies of cyanobacteria that will and metabolomics) would enhance our understanding of ultimately result in higher final product yield are being cyanobacterial biochemistry. For example, recent transcrip- explored. RuBisCo is an essential enzyme that catalyzes the tomes studies on genetically engineered Synechocystis sp. first major step of carbon fixation. However, it is a notoriously PCC 6803 provide insights into understanding the effects of inefficient enzyme that has slow catalytic rate and is subject overexpressing PHA biosynthetic genes on photoautotrophic to competitive inhibition by O2. RuBisCo is suggested to be biopolymer production [73]. 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